Bisimide resins, e.g. bismaleimide resins, are advantageously used in providing resin matrix composites, e.g. glass or carbon fiber reinforced laminates, to achieve enhanced properties such as greater thermal stability and lower moisture sensitivity than is possible with other composites, e.g. composites based on epoxy or other resins. A common bisimide, i.e. bis(4-maleimidophenyl)methane, exhibits poor solubility in many organic solvents of choice. Its use in commercial manufacture of laminates is facilitated by dissolving the bisimide in N-methyl pyrrolidone (not a preferred solvent) and by chain extension by Michael addition reaction with diamines.
Nishikawa, et al., disclose in U.S. Pat. No. 4,460,783 that certain aromatic ether bismaleimide compounds such as bis(maleimidophenoxyphenyl) propane and the like are highly soluble in desirable solvents such as acetone, toluene, methyl ethyl ketone and the like. See also Harvey et al in "New Aromatic-Ether Bismaleimide Matrix Resins", ANTEC '86, page 1311.
It has been discovered that the solubility of such aromatic ether bismaleimides is dependent on the presence of a considerable amount of acid group-containing precursor, i.e. having terminal amic acid groups not converted to the terminal imide group. Such acidic precursor material advantageously renders the bisimide soluble. However the terminal acid groups will tend to liberate water from ring closing imidization during cure of such resin. Such water will be vaporized during normal curing conditions and may tend to generate voids or blisters in fabricated articles such as laminates. Such water liberation is tolerable in some fabrication practices that can accommodate liberated water. However, in other applications it is especially desirable that bisimide resins cure with minimal liberation of water.
An object of this invention is to provide soluble maleimide resin comprising predominately bismaleimide compound and low but solubilizing levels of an acid-containing precursor.
Another object is to provide concentrated solutions of such resin compositions in common organic solvents. Yet another object is to provide such solutions with desirably long stability, for instance, for days, weeks or even longer.
Still another object is to provide such resins and solutions that rapidly form a viscous thermoplastic resin that allows for minimal resin flow from the reinforced matrix even during severe curing conditions, e.g. high pressure.